Add TrackablePointer.elapsedTime

docs/api/trackable-pointer.md

 
 The current velocity of the pointer, given in normalized units per second. You can get the speed of motion by calculating the [magnitude](vector2.md#length) of this vector.
 
+### elapsedTime
+
+`pointer.elapsedTime: number, read-only`
+
+The elapsed time, in seconds, since the tracking of this pointer began.
+
 ### isPrimary
 
 `pointer.isPrimary: boolean, read-only`

src/trackers/pointer-tracker/pointer-tracker.ts

             // determine the velocity
             const velocity = deltaPosition._clone()._scale(inverseDeltaTime);
 
+            // determine the elapsed time since the tracking began
+            const elapsedTime = previous ? previous.elapsedTime + deltaTime : 0;
+
             // determine whether or not this is the primary pointer for this type
             const isPrimary = event.isPrimary;
 
 
             // we create new trackable instances on each frame;
             // these will be exported and consumed by the user
-            this._newPointers.set(id, { id, phase, position, deltaPosition, initialPosition, velocity, isPrimary, type });
+            this._newPointers.set(id, { id, phase, position, deltaPosition, initialPosition, velocity, elapsedTime, isPrimary, type });
 
         }
 

src/trackers/pointer-tracker/trackable-pointer.ts

     /** current velocity, given in normalized units per second */
     readonly velocity: Vector2;
 
+    /** elapsed time, in seconds, since the tracking of this pointer began */
+    readonly elapsedTime: number;
+
     /** whether or not this is the primary pointer for this type */
     readonly isPrimary: boolean;